Have you found out what the problem is and if there is any fix? I just recently switched from SPMC to MrMC on the Nvidia Shield and I am having a similar audio out of sync issue but apparently only on some video files.

I set an audio ahead offset of 34 ms on my receiver and that seems to have fixed the issue with Dolby Digital+ (5.1) content. However, stereo content and Atmos have different offsets that I have to mess with every time to get lips synced

just tried clearing data / cache on the MrMC app and starting from scratch. only change I made was to enable passthrough audio settings.

I then played a Dolby Digital+ 5.1 video and the audio was out of sync. tried disabling refresh rate switching: same issue. playing the exact same video on the Shield Plex app has no sync issues (with audio passthrough enabled and refresh rate switching on even).